As conventional technique, an image coding apparatus having a following function exists. When an image is coded (encoded), a coding amount of coded data of the image is adjusted by controlling a quantization level for each processing unit (For example, each line or each block of the image) of coding. This control is called “bit rate control”.
For example, as bit rate control, CBR (Constant Bit Rate) control to nearly fix a coding amount of coded data for each processing unit is well known. However, as to CBR control, degradation of image quality locally occurs in the image.
In order to suppress local degradation of image quality, in the image coding apparatus of conventional technique, for each processing unit, a coding mode is selected based on the sum of a coding amount coded by a plurality of variable length coding modes (compression rates thereof are different) and a coding amount coded by a fixed length coding mode. Then, each processing unit is coded by the selected coding mode thereof.
However, in this image coding apparatus, when each line is coded, due to the selected coding mode, effective coding cannot be often performed.